Based on a new semi-classical theory of multi-step nuclear reaction processes the code UNF code has been developed for calculations of fast neutron data for structural materials with neutron energies below 20 MeV. Besides the elastic scattering channel the code may handle decay sequences up to the (n,3n) reaction, including 14 reaction channels.
The energy spectra and double differential cross sections of all kinds of emitted particles as well as of the recoil nuclei can be obtained, Gamma production data, KERMAX factors and production spectra of n, p, alpha, deuteron, triton, helium-3 are also given. The output form is in the ENDF-6 format.

Incident neutron energies are from 1KeV to 20 MeV. There are 3 parameters in this code: Incident neutron energies NEL, the number of gamma ray spectra NGS (<200), the number of discrete levels of residual nuclei for the first particle emissions NLV. The user can set the values of NEL and NLV according to the storage size of the computer used. The number of discrete levels of residual nuclei for the multi-particle emissions is not greater than 20.

A code for automatic adjustment of optical model parameters is needed in order to determine the optimal values of optical potential parameters as input to this code. If the direct scattering data and the direct reaction data are available from other codes, one can input the data so that the results will include the direct process effects.